Milestone 5 of 8

Implement and hand-check the 3 × 3 mean rule

Repeat nearest edge values, accumulate each channel in a wider integer dtype, divide by nine with floor division, and check center, edge, and corner pixels.

The local mean filter is a small neighborhood rule that can be calculated by hand. State its edge behavior before writing the loop.

Goal

Implement the supplied 3 × 3 mean rule for grayscale and RGB arrays and hand-check center, edge, and corner pixels.

Rule

For every pixel and channel, take the surrounding 3 × 3 values. Outside the image, repeat the nearest edge value. Add in a wider integer dtype and use floor division by 9. Return an independent uint8 array with the same shape.

This is a local averaging rule supplied for practice. It is not a learned filter, an optimal filter, or a replacement for the later lesson on convolution.

Deliverables

Implement mean_3x3(image) in src/operations.py. Save hand calculations and pixel checks for tiny grayscale and RGB fixtures, including the center, all four corners, and at least one edge pixel. Record input and output shape, dtype, range, and selected values.

Checks

Use a one-pixel image, a narrow image, a constant image, a gradient, and a checker pattern. Check edge repetition at corners and edges, channel independence, wider accumulation, and floor division. For a constant image, the output should remain constant. Check that applying the operation twice does not mutate the first result or its source.

Compare every selected pixel with a hand-computed value. Reject arrays outside the toolkit shape, dtype, or range contract. Check grayscale and RGB output shapes exactly.

Workspace

Keep the filter and its numerical fixtures in src/operations.py. Do not add convolution terminology, learned parameters, or model comparisons.

Hints

HintCount nine values

At a corner, several of the nine positions refer to the same edge value. Count each repeated position before dividing.

HintChannels do not mix

Calculate one channel independently. A red value must not affect the green or blue output at the same pixel.

Review

Calculate the output at the top-left corner of a two-by-two pattern by hand. Which source value is repeated, and why? What does this rule say about its local arithmetic, and what does it not say about visual quality?

How to check your work

Checks compare center, edge, and corner values, shape, dtype, range, and source immutability with the hand fixtures. The rule is exact and small by design.